Can i still use os x el capitan in 2021

Yes, as long as it is functioning.

One might be able to upgrade to a newer version of macOS.

The Age of this computer will Govern what newer version of macOS it will Qualify to install. Use About this Mac from the Apple Icon on Desktop - the required Information will be there.

With the above information in-hand - follow this How to get old versions of macOS and verify what version this computer Qualifies to install.  For Best results use Safari to commence the download as Others may not work.

One may consider upGrading to High Sierra first. This may better position the computer for the Drive Conversion to the newer APFS format as well as any EFI BOOT ROM required to use the newer file format.

Tips Big Sir Upgrade

Tips Monterey Upgrade 

Make Time Machine Backup  is very useful and can be used to Revert to Previous Working macOS

Notation - Mojave is the Last version to support 32 bit applications / extensions and Drivers. Catalina, Big Sur & Monterey supports only 64 bit and there is no workaround.

Suggest using Go64 to find 32 bit application before attempting the upGrade. Some or many of the Application already installed may required update or upGrades to 64 Bit Versions.

Are there any AntiVirus, Disk Cleaner, Optimizers, Un-installers, etc installed which should be removed as per Developers Instructions. They are useless, unneeded, cause havoc and interfere with the normal operation of the OS and may even Corrupt the OS requiring a Reinstallation

Im running El Capitan on a 2009 Mac Mini without any issue. Im logged into iCloud, but I only sync photos and use Mail. Everything works for what I need, Im also partitioned for Windows XP.

My 2012 MacBook Pro (last generation with the matte screen) (in mint condition) I'm running Catalina. I also have a 2014 Mac Mini running Catalina 10.15.17.

I do NOT want a replacement